Geology of the Southern Dora-Maira Massif: Insights from a Sector with Mixed Ophiolitic and Continental Rocks (Valmala Tectonic Unit, Western Alps)
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Journal of Maps ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/tjom20 Geology of the southern Dora-Maira Massif: insights from a sector with mixed ophiolitic and continental rocks (Valmala tectonic unit, Western Alps) Gianni Balestro , Francesco Nosenzo , Paola Cadoppi , Gianfranco Fioraso , Chiara Groppo & Andrea Festa To cite this article: Gianni Balestro , Francesco Nosenzo , Paola Cadoppi , Gianfranco Fioraso , Chiara Groppo & Andrea Festa (2020) Geology of the southern Dora-Maira Massif: insights from a sector with mixed ophiolitic and continental rocks (Valmala tectonic unit, Western Alps), Journal of Maps, 16:2, 736-744 To link to this article: https://doi.org/10.1080/17445647.2020.1824825 © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of Journal of Maps View supplementary material Published online: 08 Oct 2020. Submit your article to this journal View related articles View Crossmark data Full Terms & Conditions of access and use can be found at https://www.tandfonline.com/action/journalInformation?journalCode=tjom20 JOURNAL OF MAPS 2020, VOL. 16, NO. 2, 736–744 https://doi.org/10.1080/17445647.2020.1824825 Science Geology of the southern Dora-Maira Massif: insights from a sector with mixed ophiolitic and continental rocks (Valmala tectonic unit, Western Alps) Gianni Balestro a, Francesco Nosenzob , Paola Cadoppia, Gianfranco Fiorasoc, Chiara Groppoa and Andrea Festaa aDepartment of Earth Sciences, University of Torino, Torino, Italy; bDepartment of Geological Sciences, Stockholm University, Stockholm, Sweden; cInstitute of Geoscience and Earth Resources, National Research Council, Torino, Italy ABSTRACT ARTICLE HISTORY In the Valmala sector of the southern Dora Maira Massif (Western Alps), two different eclogite- Received 16 April 2020 and blueschist-facies units (i.e. the Rocca Solei and Dronero units, respectively), are separated Revised 8 September 2020 by a shear zone (i.e. the Valmala Tectonic Unit), which peculiarly consists of mixed slices of Accepted 14 September ophiolitic and continental rocks. A detailed geological map at 1:10,000 scale allowed to 2020 ‘ ’ point out that the tectonic slices within the Valmala Tectonic Unit consist of native rock KEYWORDS ‘ ’ slices wrenched from the overlying Dronero Unit, and exotic rocks likely sourced from Alpine tectonics; Dora-Maira other units of the Dora Maira and from a continental margin and an oceanic basin. On the Massif; tectonic mélanges; contrary, rock slices sourced from the underlying Rocca Solei Unit are lacking. The overall ophiolites; shear zone tectonic stack results after an early subduction-related deformation phase (i.e. the D1), and the pervasive overprinting of two subsequent exhumation-related deformation phases (i.e. the D2 and D3). The Valmala Tectonic Unit is inferred to have played a role in decoupling the southern Dora Maira Massif during subduction, and/or in driving exhumation of the ultra-high pressure rocks occurring in the adjoining Brossasco-Isasca Unit. 1. Introduction stratigraphic and structural setting of the whole DM are scattered and actually not exhaustive. The Western Alps represent one of the more studied Throughout a detailed geological map at 1:10,000 scale orogens in the world. Nevertheless, a large part of (see Main Map), this paper provides new detailed infor- this orogen lacks of detailed geological maps and the mation about the geology of a poorly known tectonic official Geological Maps of Italy at 1:100,000 scale stack in the southern DM (i.e. the Valmala sector; Figure are older than 50 years. 1), which lies above the coesite-eclogite bearing Brossaco- During the last tens of years, several works based on Isasca Unit (Chopin et al., 1991; Kienastetal.,1991). In a detailed geological mapping, clearly documented that this sector, two different DM units (i.e. the eclogite-facies the Western Alps are characterized by complex struc- Rocca Solei Unit and the blueschist-facies Dronero Unit) tural and tectono-stratigraphic settings, part of which are separated by a tectonic unit (i.e. the here defined Val- results from pre-orogenic structural inheritances (see mala Tectonic Unit), which peculiarly consists of mixed e.g. Ballèvre et al., 2018; Bell & Butler, 2017; Festa slices of ophiolitic and continental rocks. et al., 2020; Mohn et al., 2011; Tartarotti et al., 2017, and reference therein), superposition of subduction- and exhumation-related deformation stages (see e.g. 2. Regional setting Federico et al., 2015; Manzotti et al., 2014; Roda et al., 2018, and reference therein) and late strike-slip to The Western Alpine orogenic belt developed as a result extensional tectonics (see e.g. Balestro et al., 2009; Per- of the collision between Adria, in the upper plate, and rone et al., 2010, and reference therein). This is the case Europe, in the lower plate, after the closure of the inter- of the Dora-Maira Massif (DM hereafter), a terrane of posed Ligurian–Piedmont oceanic basin (see e.g. Cow- continental crust, worldwide known for the first dis- ard & Dietrich, 1989; Rosenbaum & Lister, 2005; covery of coesite-bearing ultra-high pressure mineral Schmid et al., 2017, and references therein). The belt assemblages (Chopin, 1984). Despite several meta- evolved through the (i) Late Cretaceous to Middle morphic and petrological studies focused on the Eocene subduction, (ii) Late Eocene to Early Oligocene ultra-high pressure Brossasco-Isasca Unit (see e.g. continental collision, and (iii) Late Oligocene to Neo- Compagnoni et al., 2012; Ferrando et al., 2017, and gene deep crust/mantle indentation (see e.g. Solarino reference therein), published data about the tectono- et al., 2018, and references therein). The DM represents CONTACT Gianni Balestro [email protected] Department of Earth Sciences, University of Torino, Via Valperga Caluso 35, 10125 Torino, Italy © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of Journal of Maps This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrest- ricted use, distribution, and reproduction in any medium, provided the original work is properly cited. JOURNAL OF MAPS 737 Figure 1. Tectonic map of the Western Alps (modified after Balestro et al., 2015). The red square indicates the study area. a slab of the paleo-European continental margin, which Sampeyre units (Chopin et al., 1991; Groppo et al., was deeply involved in the subduction- and exhuma- 2019), consisting of polymetamorphic mica schists, tion-related Alpine tectonometamorphic processes. It monometamorphic meta-intrusive bodies, Permian is now stacked in the inner sector of the Western metasediments locally with interlayered metavolcanic Alps (i.e. the Upper Penninic Domain; Dal Piaz et al., rocks and Lower Triassic siliciclastic metasediments 2003), and tectonically overlain by different meta- (Boniolietal.,1992;Michard&Vialon,1966).Inbetween ophiolite complexes (see e.g. Balestro et al., 2014, 2018). the eclogite-facies units and the blueschist-facies Dro- The DM consists of different tectonic units (i.e. the nero and Sampeyre units, meta-ophiolitic rocks consist- different ‘Ensembles’ of Vialon, 1966, and lithostrati- ing of serpentinite, metabasite and calcschist graphic units of Sandrone et al., 1993), which differ discontinuously occur (i.e. the ‘ophiolitiferous band’ of from each other for both their lithostratigraphy, Alpine Henry et al., 1993;seealsoCarraro et al., 1971). In the metamorphic P-T peaks and structural positions (Figure southernmost sector of the DM, the Dronero and Sam- 1). The lowermost DM tectonic unit corresponds to the peyre units are tectonically overlain by a succession of PineroloUnit,atectonicwindowoccurringinthecentral carbonate metasediments (pre-Piedmont Auct.; sector of the DM. It was metamorphosed under blues- Michard, 1967), which was detached from the European chist-facies peak conditions (Avigad et al., 2003; Borghi distal margin (Figure 1). This succession mainly consists etal.,1984),anditconsistsofUpperCarboniferousmeta- of Middle-Late Triassic platform-derived dolomitic sediments,whichwereintrudedbyLowerPermiandiori- marble,andofEarly-Jurassicsyn-riftmarbleandcarbon- tic and granitic melts (Bussy & Cadoppi, 1996; Manzotti ate-rich calcschist with layers of carbonate metabreccia. etal., 2016; Perrone et al., 2011). The Pinerolo Unit istec- The structural setting and tectonic evolution of the tonically overlain by different eclogite-facies tectonic southern DM is related to three main stages (see e.g. units (see e.g. Compagnoni & Rolfo, 2003; Gasco et al., Henry et al., 1993), which correspond to the D1, D2 2011), consisting of a composite polymetamorphic base- and D3 phases. The subduction-related D1 was coeval ment, intruded by Lower Permian granitoid bodies, and to the high-pressure and ultra-high pressure meta- of discontinuous monometamorphic successions of sili- morphism, whereas the exhumation-related D2 and ciclasticandcarbonatemetasediments,PermiantoTrias- D3 were coeval to the widespread greenschist-facies sic in age (Bussy & Cadoppi, 1996; Sandrone et al., 1993). retrograde recrystallization (see e.g. Groppo et al., In the southern sector of the DM, these units correspond 2007; Henry et al., 1993). from bottom to top to the eclogite-facies San Chiaffredo Unit, the coesite-eclogite facies Brossasco-Isasca Unit 3. Methods and the eclogite-facies Rocca Solei